This invention relates to the field of communication electronics, and more particularly, to a spread spectrum transceiver for use in wireless local area networks.
Wireless or radio communication between separated electronic devices is widely used. For example, a wireless local area network (WLAN) is a flexible data communication system that may be an extension to, or an alternative for, a wired LAN within a building or campus. A WLAN uses radio technology to transmit and receive data over the air, thereby reducing or minimizing the need for wired connections. Accordingly, a WLAN combines data connectivity with user mobility, and, through simplified configurations, also permits a movable LAN.
Over the past several years, WLANs have gained acceptance among a number users including, for example, health-care, retail, manufacturing, warehousing, and academic areas. These groups have benefitted from the productivity gains of using hand-held terminals and notebook computers, for example, to transmit real-time information to centralized hosts for processing. Today WLANs are becoming more widely recognized and used as a general purpose connectivity alternative for an even broader range of users. In addition, a WLAN provides installation flexibility and permits a computer network to be used in situations where wireline technology is not practical.
In a typical WLAN, an access point is provided by a transceiver, that is, a combination transmitter and receiver, connects to the wired network from a fixed location. Accordingly, the access transceiver receives, buffers, and transmits data between the WLAN and the wired network. A single access transceiver can support a small group of collocated users within a range of less than about one hundred to several hundred feet. The end users connect to the WLAN through transceivers which are typically implemented as PC cards in a notebook computer, or ISA or PCI cards for desktop computers. Of course the transceiver may be integrated with any device, such as a hand-held computer.
Spread spectrum communications have been used for various applications, such as cellular telephone communications, to provide robustness to jamming, good interference and multi-path rejection, and inherently secure communications from eavesdroppers, as described, for example, in U.S. Pat. No. 5,515,396 to Dalekotzin. The patent discloses a code division multiple access (CDMA) cellular communication system using four Walsh spreading codes to allow transmission of a higher information rate without a substantial duplication of transmitter hardware. U.S. Pat. No. 5,535,239 to Padovani et al., U.S. Pat. No. 5,416,797 to Gilhousen et al., U.S. Pat. No. 5,309,474 to Gilhousen et al., and U.S. Pat. No. 5,103,459 to Gilhousen et al. also disclose a CDMA spread spectrum cellular telephone communications system using Walsh function spreading codes. The disclosures of each of these patents is hereby incorporated by reference in their entirety.
It is possible to reduce multipath problems, such as with GSM cell phones using a Viterbi maximum likelihood sequence estimation (MLSE). However, applying MLSE is complex for 802.11 high data rate receivers. The Viterbi algorithm, nonetheless, if it could be applied to 802.11 spread spectrum transceivers, could provide a beneficial way to attack the multipath problem, especially if used in combination with other sources of equalization.
Viterbi encoders and detectors have been used in various electronic and digital uses, including digital communication channels, such as disclosed in U.S. Pat. No. 5,774,504, and the systems and circuits as disclosed in U.S. Pat. Nos. 5,841,819 and 5,844,741, the disclosures which are hereby incorporated by reference in their entirety.
The assignee of the present invention has used Viterbi algorithm and decoders, such as disclosed in U.S. Pat. Nos. 4,545,054 and 4,346,473, the disclosures which are hereby incorporated by reference in their entirety.
It is therefore an object of the present invention to provide a spread spectrum transceiver having a base band processor and demodulation that may use the Viterbi algorithm with feedback equalization.
In accordance with the present invention, a method of the present invention demodulates a spread spectrum signal and comprises the steps of receiving within a demodulator of a spread spectrum receiver, a spread spectrum phase shift keyed (PSK) modulated information signal on a signal channel. The information signal includes a sequence of data symbols formed from a plurality of high rate mode chips. The method also comprises the step of Viterbi decoding a precursor portion of the signal channel and forming a multi-state trellis having a predetermined number of states. The method also comprises the step of feedback equalizing a post-cursor portion of the signal channel with a finite impulse response filter having feedback taps operatively connected to a chip detector circuit that tracks high rate mode chips and a carrier loop circuit for phase and frequency tracking. The method also comprises the step of despreading the information signal within the spread spectrum code function correlator.
In still another aspect of the present invention, the method comprises the step of performing a partial signal trace back of the multi-state trellis to the feedback taps of the finite impulse response filter. The method also comprises the step of passing a signal trace back from a Viterbi detector circuit to the spread spectrum code function correlator. The method also includes a sequence of data symbols formed from a plurality of high rate mode chips. The signal channel forms a finite state machine and further comprises the step of establishing a predetermined number of states within the multi-state trellis.
In accordance with the present invention, a spread spectrum radio transceiver is disclosed and includes a base band processor and radio circuit connected thereto. The base band processor includes a demodulator for spread spectrum phase shift keying (PSK) demodulating an information signal on a signal channel. The information signal includes data symbols formed from a plurality of high rate mode chips forming a spread spectrum information signal. The demodulator comprises at least one spread spectrum code function correlator for decoding the information according to a predetermined orthogonal code sequence. A Viterbi decoder receives the information signal and decodes a precursor portion of the signal channel. A carrier loop circuit allows for phase and frequency tracking of the information signal. A chip decision circuit is operative with a carrier loop circuit and tracks high rate chips.
A decision feedback equalizer is formed from a finite impulse response filter and operative with the chip decision circuit and carrier loop circuit. This equalizer has a plurality of feedback taps and at least one feedback tap is selected for logical add/subtract operations for equalizing a post-cursor portion of the signal channel. A modulator spread spectrum PSK modulates information for transmission via the radio circuit. The modulator comprises at least one code function encoder for encoding information according to an orthogonal code sequence. The at least one spread spectrum code function correlator comprises a Walsh correlator. The Viterbi decoder can further comprise means for generating a trace back soft decision to the spread spectrum code function correlator. The Viterbi decoder can further comprise means for generating a partial trace back to predetermined feedback paths of the finite impulse response filter. The carrier loop circuit can also include a carrier numerically controlled oscillator (NCO). A carrier NCO control circuit selectively operates the carrier NCO based upon a carrier phase of the carrier loop circuit.